1. Field of the Invention
The present invention relates generally to Power over Ethernet (PoE) and, more particularly, to transmission impedance for midspan powering in a PoE application.
2. Introduction
The IEEE 802.3af and 802.3at PoE specifications provide a framework for delivery of power from power sourcing equipment (PSE) to a powered device (PD) over Ethernet cabling. In this framework, various PDs can be deployed such as voice over IP (VoIP) phones, wireless LAN access points, network cameras, computing devices, etc.
In the PoE process, a valid device detection is first performed. This detection process identifies whether or not it is connected to a valid device to ensure that power is not applied to non-PoE capable devices. After a valid PD is discovered, the PSE can optionally perform a Layer 1 power classification.
In one configuration, power can be sourced directly from data terminal equipment such as an Ethernet switch. This type of network device is referred to as an endspan device. In another configuration, power can be supplied by a midspan device. FIG. 1 illustrates such a midspan configuration, which includes a medium dependent interface (MDI) link between Ethernet switch 110 and PD 130. Midspan configurations can be used for various systems such as 100BASE-TX and 1000BASE-T systems. As illustrated, midspan PSE 120 is placed in the middle of the MDI link for the insertion of power to be supplied to PD 130. An advantage of such a midspan configuration is the usage of legacy Ethernet switches that do not have PoE capabilities.
In one midspan configuration that supports 100BASE-TX, data is transmitted using pins 1 (TX+), 2 (TX−), 3 (RX+), and 6 (RX−) of the Ethernet cable. The remaining pins 4, 5, 7, and 8, which are representative of the two unused pairs of the Ethernet cable, can be used by midspan PSE 120 for the transmission of power to PD 130. In an alternative midspan configuration, power is not transmitted on the two unused pairs, but transmitted on the two signal pairs used for data transmission.
Midspan PSE design guidelines have not been tightly specified beyond a general recognition that the cabling channel performance should be maintained by the midspan PSE when it is inserted into an MDI link. What is needed therefore is a mechanism that ensures that a midspan PSE maintains proper compatibility with various network elements such as legacy Ethernet devices.